feat: stat distribution UI + rest button + xpToNextLevel from backend

Dashboard: stat distributor with +/- buttons when statPoints > 0,
rest button (+50% HP, -20 endurance) when HP < max,
XP bar uses xpToNextLevel from backend instead of local formula.
API: distributeStats + rest endpoints added to client.

frontend/src/api/endpoints.ts

@@ -17,6 +17,9 @@ export const characterApi = {
   create: (name: string, stats: Record<string, number>) =>
     api.post<Character>('/characters', { name, ...stats }),
   me:     () => api.get<Character>('/characters/me'),
+  distributeStats: (stats: Record<string, number>) =>
+    api.post<Character>('/characters/stats', stats),
+  rest:   () => api.post<{ hpBefore: number; hpAfter: number; hpMax: number; healed: number }>('/characters/rest'),
 };
 
 // Combat